1. Field of the Invention
The present invention relates to semiconductor pressure detecting devices and, more particularly, to a semiconductor pressure detecting device which uses, as its detector element, a semiconductor sensor element capable of detecting a strain and/or stress that occurs to a thin-walled pressure receiving portion.
2. Discussion of the Background
Conventionally, as one type of pressure detecting device, there has been known one which uses, as its detector element, a semiconductor sensor element (hereinafter, abbreviated simply as sensor element when appropriate) capable of detecting a strain and/or stress that occurs to a thin-walled diaphragm-like pressure receiving portion by making use of the semiconductors' piezoresistance effect (see, for example, Japanese Patent Laid-Open Publication SHO 63-196826).
Using this type of sensor element makes it possible to detect with high accuracy the magnitude and/or change of a pressure that acts on the pressure receiving portion as a magnitude and/or change of a strain and/or stress and then to convert the detection result into an electric signal as an output.
Such a semiconductor pressure detecting device requires not only the semiconductor sensor element having a gauge resistor but also various peripheral circuits such as a resistor circuit for adjusting electrical characteristics of the sensor element. These peripheral circuits and the like, in recent years, tend to be incorporated and integrated into the sensor element itself.
In this case, the size (in particular, planar size) of a semiconductor sensor element would become larger than when the peripheral circuits and the like are formed on a separate board. This in turn would cause component members (later-described pedestal seat and base member) that support the sensor element to be inevitably increased in size as well.
FIG. 9 is a longitudinal cross-sectional explanatory view showing the basic construction of a semiconductor pressure detecting device (hereinafter, abbreviated simply as pressure detecting device or device when appropriate) according to the prior art.
As shown in FIG. 9, this pressure detecting device 101 according to the prior art comprises a semiconductor sensor element 102 made of silicon (Si) single crystal and having at a central portion a thin-walled diaphragm-like pressure receiving portion 102a, a pedestal seat 103 made of silicon for joining and supporting the sensor element 102, and a metallic base member 104 for joining and supporting the pedestal seat 103. A pressure introducing pipe 105 is joined to the base member 104.
In the semiconductor sensor element 102, not only a gauge resistor but also peripheral circuits such as a resistor circuit for adjusting the electrical characteristics of the sensor element 102 are incorporated and integrated. Accordingly, the sensor element 102 as well as the pedestal seat 103 and the base member 104 for supporting the sensor element 102 are increased in size (particularly, planar size) as compared with the case in which peripheral circuits and the like are formed on a separate board.
The base member 104 has a body portion 104a for joining and supporting the pedestal seat 103 on its top side, and a thin-walled flange portion 104b provided on its outer peripheral side. This flange portion 104b is formed integrally with the body portion 104a by compression molding an outer peripheral portion of the body portion 104a with a press. To this flange portion 104b of the base member 104, is joined a flange portion 111c provided at an opening end of a metallic cap 111.
To the body portion 104a of the base member 104, are fixed a plurality of lead wires 113 that are inserted and led thicknesswise through the base member 104. The lead wires 113 are electrically connected to the sensor element 102 via wires 112 made of, for example, gold (Au).
Also, the pedestal seat 103 has a pair of fitting plates 103a, 103b which are opposed to each other with an annular recess 103g therebetween. The sensor element 102 is joined to the upper surface of the upper fitting plate 103b, while the lower surface of the lower fitting plate 103a is joined to the upper surface of the body portion 104a of the base member 104.
The sensor element 102 and the pedestal seat 103, as well as the pedestal seat 103 and the base member 104 are joined to each other, respectively, by the so-called die bonding process so as to be sealed airtight and fluid-tight. Besides, by the pressure introducing pipe 105 being joined to the lower surface of the base member 104, an internal passage of the pressure introducing pipe 105 and pressure introducing holes formed in respective central portions of the base member 104 and the pedestal seat 103 are communicated with one another in succession so that fluid is introduced to a pressure chamber 109 formed between an inner wall of the sensor element 102 including the pressure receiving portion 102a and the upper surface of the pedestal seat 103.
In addition, a vacuum chamber 110 is formed by a space defined by an outer surface of the unit body comprised of the sensor element 102, the pedestal seat 103 and the base member 104, and an inner wall surface of the cap 111.
In the pressure detecting device 101 as described above, the flange portion 111c of the cap 111 and the flange portion 104b of the base member 104 are joined together by welding in the final assembly process. Because of a strain and/or stress that occurs to the surface of the body portion 104a of the base member 104 due to the welding, there arises a crack Cr (see broken line in FIG. 9) in the silicon pedestal seat 103 so that the degree of vacuum of the vacuum chamber 110 decreases, or otherwise, even without the occurrence of the crack Cr, the sensor element 102 undergoes an effect of welding strain so that its pressure characteristics are varied, resulting in deteriorated detection accuracy, as a problem of the pressure detecting device 101.
The strain that occurs to the surface of the base member 104 because of the welding of the flange portions 104b, 111c with each other increases more and more with increasing size of the base member. Accordingly, when peripheral circuits are incorporated into the semiconductor sensor element 102, in which case the planar size of the base member 104 has been increased proportionally, the problem of welding strain of the base member 104 also increases.
In conjunction with this problem, the present inventor has found out as a result of energetic studies that the strain that occurs to the surface of the body portion of the base member 104 becomes a maximum at a center portion of the base member, whereas the thinner the flange portion 104b of the base member 104, the smaller the amount of strain, and that particularly when the thickness of the flange portion 104b is substantially equal to or less than the thickness of the flange portion 111c of the cap 111, the effect of the welding strain can be reduced to a very small one.
However, with respect to the thicknesses of the two flange portions 111c, 104b, since the flange portion 111c of the cap 111 is formed integrally with the cap 111, the setting range of its thickness would be restricted by the thickness of the cap 111 itself. Besides, since the flange portion 104b of the base member 104 is also integrally formed by compression molding an outer portion of the body portion 104a, an attempt to thin the flange portion 104b would be restricted by its wall thickness ratio to the thickness of the body portion 104a.
The base member 104 is a fundamental component that supports main part (pedestal seat 103 and sensor element 102) of the pressure detecting device 101, and so needs to be directly burdened with not only the thermal stress due to welding as stated above but also stress due to any load applied external thereto. Accordingly, it is important to enhance the rigidity of the base member 104 (in particular, rigidity of the portion that joins and supports the pedestal seat 103) from the point of view of improving the durability of the pressure detecting device.